1. Field of the Invention
The present invention relates to a substrate holding apparatus, and more particularly to a substrate holding apparatus in a chemical mechanical polishing apparatus for polishing a substrate such as a semiconductor wafer to a flat mirror finish.
2. Description of the Related Art
As semiconductor devices have become more highly integrated in recent years, circuit interconnections have become finer and distances between those circuit interconnections have become smaller. In a case of photolithography, which can form interconnections that are at most 0.5 μm wide, it is required that surfaces on which pattern images are to be focused by a stepper should be as flat as possible because depth of focus of an optical system is relatively small. In order to planarize such a semiconductor wafer, there has been used a polishing apparatus for performing chemical mechanical polishing (CMP).
This type of chemical mechanical polishing apparatus comprises a polishing table having a polishing pad (polishing cloth) attached to an upper surface of the polishing table, and a top ring for holding a substrate to be polished, such as a semiconductor wafer. The polishing table and the top ring are rotated at independent rotational speeds, respectively. The top ring presses the substrate against the polishing pad under a predetermined pressure. A polishing liquid (slurry) is supplied from a polishing liquid supply nozzle onto the polishing pad. Thus, a surface of the substrate is polished to a flat mirror finish.
FIG. 1 is a schematic view showing a main portion of a conventional polishing apparatus. The conventional polishing apparatus includes a rotatable polishing table (turntable) 102 having a polishing pad (polishing cloth) 100 attached to an upper surface of the polishing table 102, a top ring 104 for holding a semiconductor wafer (substrate) W, and a polishing liquid supply nozzle 106 for supplying a polishing liquid Q to the polishing pad 100. The top ring 104 is configured to rotate the semiconductor wafer W and press the semiconductor wafer W against the polishing pad 100. The top ring 104 is connected to a top ring shaft 108, which is vertically movable via an air cylinder provided in a top ring head (not shown).
The top ring 104 has an elastic pad 110 attached to a lower surface of the top ring 104. For example, the elastic pad is made of polyurethane. The semiconductor wafer W is held by the top ring 104 in a state such that the semiconductor wafer W is brought into contact with the elastic pad 110. Further, the top ring 104 has a cylindrical guide ring 112 provided at a peripheral portion of the top ring 104. The guide ring 112 serves to prevent the semiconductor wafer W from being separated from the lower surface of the top ring 104 during polishing. The guide ring 112 is fixed to the peripheral portion of the top ring 104. The guide ring 112 has a lower end located at a position lower than a holding surface of the top ring 104, and accordingly, forms a recessed portion at an inward position of the guide ring 112. Thus, a semiconductor wafer W to be polished is held within the recessed portion of the top ring 104 so as not to be ejected from the top ring 104 during polishing.
With the conventional polishing apparatus, the semiconductor wafer W is held on a lower surface of the elastic pad 110 in the top ring 104 and pressed against the polishing pad 100 on the polishing table 102 by the top ring 104. The polishing table 102 and the top ring 104 are rotated so as to move the polishing pad 100 and the semiconductor wafer W from the polishing liquid supply nozzle 106. For example, a suspension of fine abrasive particles in an alkali solution is used as the polishing liquid. Thus, the semiconductor wafer W is polished to a flat mirror finish by a combined effect of a chemical polishing effect attained by the alkali and a mechanical polishing effect attained by the abrasive particles.
In the aforementioned polishing apparatus, various complicated factors which may affect an amount of polishing should be controlled in order to planarize a semiconductor wafer W over an entire surface thereof with high accuracy. Such factors include a relative sliding speed between the semiconductor wafer W and the polishing table 102, an amount (or distribution) of polishing liquid at an interface (polishing interface) between the semiconductor wafer W and the polishing pad 100, a pressing force applied to the semiconductor wafer W to press the semiconductor wafer W against the polishing pad 100 by the top ring 104, a temperature of the polishing interface, and the like.
When an acid or alkali polishing liquid is used to perform a chemical mechanical polishing process (CMP), the temperature of the polishing interface exerts a great influence on a polishing rate. Additionally, if the top ring 104 is increased in temperature, the top ring 104 is deformed so as to exert an adverse influence on a pressing force. Thus, control of the temperature of the polishing interface has two aspects with respect to a level of planarization.
An example of a substrate holding apparatus including a means for controlling a temperature of a polishing interface is disclosed in Japanese laid-open patent publication No. 2000-225559, relevant parts of which are hereby incorporated by reference. Specifically, the substrate holding apparatus includes a holding plate having a substrate holding surface and an elastic pad attached to the holding surface of the holding plate. A substrate is held via the elastic pad on the substrate holding surface, and a surface of the substrate to be polished is pressed against a polishing surface on a polishing table. Grooves are formed in the substrate holding surface of the holding plate. The grooves are supplied with a fluid controlled in terms of temperature. Thus, a temperature of the substrate is controlled via the elastic pad.
FIG. 2 is a cross-sectional view showing a substrate holding apparatus having fluid communication grooves 128 for temperature control as disclosed in Japanese laid-open patent publication No. 2000-225559. In FIG. 2, components are illustrated in a simplified manner for easy understanding. As shown in FIG. 2, the substrate holding apparatus has a top ring 104 for holding a wafer W to be polished, an elastic pad 110 attached to a lower surface of the top ring 104, and pressure control units (regulators) 125 for controlling a pressure of a temperature control fluid 129. A plurality of communication grooves 128 are formed in the top ring 104. The temperature control fluid 129 is supplied through the pressure control unit 125 into the communication grooves 128 so as to cool a lower surface of the top ring 104. Because a cooling effect is transmitted to the wafer W via the elastic pad 110 by heat conduction, the cooling effect is adversely reduced by the elastic pad 110.
Further, no flow regulating valve is provided downstream of the communication grooves 128, but the regulators 125 are provided downstream of the communication grooves 128. Accordingly, the temperature control fluid 129 is supplied to the communication grooves 128 only when pressure of a fluid in the communication grooves 128 is lowered. Thus, the cooling effect is unsatisfactory in the conventional substrate holding apparatus. Furthermore, because the temperature control fluid 129 may leak from a periphery of the elastic pad 110 into the wafer W to cause uneven temperature control and contamination of the wafer W, a liquid cannot be used as the temperature control fluid 129.
Additionally, it is necessary to form a plurality of communication grooves 128 in the top ring 104 and further form a plurality of passages for allowing the temperature control fluid 129 to flow through the communication grooves 128. Thus, many troublesome processes are required to form grooves and holes, and a structure of the top ring 104 becomes complicated. Accordingly, use of the communication grooves 128 is effective rather in applying a back pressure to a rear face of the wafer W.
In order to solve the above drawbacks, there has been proposed a substrate holding apparatus having an air bag as shown in FIG. 3. The substrate holding apparatus has a top ring 104 for holding a wafer W on a lower surface thereof, a top ring shaft 108 for rotating the top ring 104, and an air bag 124 provided in the top ring 104 in a manner such that the wafer W is brought into contact with the air bag 124. The air bag 124 is supplied with a fluid 129 controlled in terms of temperature through a regulator 125. While the substrate holding apparatus shown in FIG. 2 controls a temperature of the wafer W via the elastic pad 110, the substrate holding apparatus shown in FIG. 3 controls a temperature of a rear face of the wafer W directly by temperature control fluid 129 in the air bag 124. The air bag 124 in this substrate holding apparatus is mainly used to uniformly press the wafer W. Specifically, the regulator 125 is employed in order to adjust pressure of the air bag 124 at a predetermined value. Accordingly, fluid 129 flows into the air bag 124 only when an internal pressure of the air bag 124 is lower than the predetermined value.
Although the substrate holding apparatus shown in FIG. 3 is configured to directly control a temperature of the wafer, no attention is paid to discharge of the temperature control fluid 129 from the air bag 124. Accordingly, the temperature control fluid 129 above the rear face of the wafer W is not replaced and thus serves merely as a fluid for applying a back pressure to control a pressing force of the wafer W. As a result, the wafer W is increased in temperature according to a polishing process. In a case of a copper CMP (chemical mechanical polishing), if temperature of a wafer becomes greater than a certain value, a polishing rate is lowered, particularly, at a central portion of the wafer.